[RFC] Early flush: new, improved (updated)

This is an incremental update to the early flush patch. Changes: - flush_dirty_buffers returns sectors flushed instead of buffers flushed - submitted_sectors now treated as signed for robustness - misc cleanups To do: - Add retired_sectors, redefine queued_sectors in terms of submitt

[RFC] Early flush: new, improved

The early-flush patch I posted a couple of days ago had the virtue of being simple and functional, but not optimal for all loads, particularly sporadic loads that are neither continously heavy or light. Today's patch is not a lot more complex, but works quite a lot better. The new kflush algo